This invention relates to a signal transmission circuit for carrying out signal transmission between an antenna unit such as a GPS (global positioning system) antenna and a receiver body.
As well known in the art, a GPS receiver is an apparatus for detecting a current position of a mobile station for a user by receiving electric waves radiated on earth from a plurality of GPS (global positioning system) satellites which go over the earth.
As well known in the art, the GPS (global positioning system) is a satellite positioning system using military satellites under U.S. Department of Defense control that comprises twenty-four non-geostationary satellites in total, with six orbit surfaces every four satellites at an orbit height of about 20,000 km. The above-mentioned non-geostationary satellites (military satellites) are called GPS satellites. If the GPS receiver receives electric waves from four GPS satellites, it is possible to carry out a three-dimensional positioning. If the GPS receiver receives electric waves from three GPS satellites, it is possible to carry out a two-dimensional positioning.
In other words, the GPS is a global positioning system comprising twenty-four artificial satellites launched by the U.S. Department of Defense, a control station on earth, and mobile stations for users. By using the global positioning system, it is possible to calculate a position, a moving direction, and a moving speed of the mobile station by measuring distances between the mobile station and three or more GPS satellites on the basis of time intervals taken for arrival of the electric waves. Although the global positioning system was originally used for military affairs, presently it is widely applied to car navigation systems or the like. In addition, the mobile stations may be not only automobiles but also airplanes, ships, or the like.
Now, xe2x80x9ccar navigationxe2x80x9d means to provide a driver information by displaying a position of a driver""s driving car on a map of a car mounted machine at a real time, by displaying road traffic information, and by calculating the most suitable route up to a driver""s destination.
Now, inasmuch as an electric wave called a GPS signal, which is generated by the GPS satellite and is arrived on the ground, has a very weak strength, the GPS signal may be buried in or covered with noises of electric waves on the ground. Accordingly, as the GPS signal, a PSK (Phase Shift Keying) wave which is spread spectrum modulated by using a PN (pseudo noise) code is used, and the GPS receiver comprises a LNA (low noise amplifier) circuit for extracting the GPS signal from the noises and for amplifying an extracted GPS signal.
Attention will be directed to the car navigation system where the mobile station is a car or an automobile. In this event, a GPS antenna (or an antenna unit) is mounted on an outer surface of a body of the car by using magnets or the like. Specifically, it will be assumed that the GPS antenna (or the antenna unit) is a planar-type antenna. The planar-type antenna may be mounted on a metallic roof panel of the car or the like by magnetically attracting the planar-type antenna to the metallic roof panel. The GPS antenna (or the antenna unit) comprises an antenna element and a circuit board on which accompanied circuit elements including the above-mentioned LNA circuit are mounted. Received by the GPS antenna (the antenna unit), a signal is transmitted to a GPS receiver body installed or loaded in the car or the automobile through a signal transmission circuit.
In prior art, transmission of a signal from the GPS antenna (the antenna unit) to the GPS receiver body (which will be merely called xe2x80x9csignal transmissionxe2x80x9d) is carried out through a coaxial cable. In other words, a conventional signal transmission circuit comprises the coaxial cable.
Inasmuch as the conventional signal transmission circuit comprises the coaxial cable as described above, the conventional signal transmission circuit is disadvantageous in that it is necessary to lead the coaxial cable from the GPS antenna (the antenna unit) to the GPS receiver body through a gap in the car or the automobile and it results in requiring a great deal of time.
In addition, those skilled in the art hit on an idea of a method comprising the step of carrying out the signal transmission by using only the above-mentioned coaxial cable as the signal transmission circuit through a glass plate of the car (for example, a front glass plate, a rear glass plate, or the like) without passing through the gap of the car. However, inasmuch as the coaxial cable has a low characteristic impedance of about 50 ohms, it is necessary to lower an impedance of its ground. As a result, an area of the ground in the glass plate must be made wide (large). To give an actual example, although the area of the ground differs from one glass plate to another, the area of the ground is equal, for example, to fifteen centimeters square, four centimeters square, or the like.
It is therefore an object of the present invention to provide a signal transmission circuit which is capable of carrying out signal transmission through a glass plate.
It is another object of the present invention to provide a signal transmission circuit of the type described, which is capable of reducing an area of a ground.
Other objects of this invention will become clear as the description proceeds.
The present inventors have been made extensive studies and considered various ideas in order to achieve a structure which needs not to widen (enlarge) the area of the ground in the glass plate on carrying out signal transmission through the glass plate. As described above, inasmuch as an unbalanced line such as a coaxial cable has low characteristic impedance of about 50 ohms, it is difficult to narrow (reduce) the area of the ground in the glass plate. Compared with this, inasmuch as a balanced line has higher characteristic impedance of, for example, about 200 ohms than that of the coaxial cable, it is possible to narrow (reduce) an area of a ground in the glass plate in comparison with a case of the coaxial cable. Accordingly, the present inventors arrived at a conclusion that it is possible to reduce the area of the ground in the glass plate by carrying out signal transmission between the glass plate using the balanced line having the high characteristic impedance and by using a balun in order to match between the unbalanced line (the coaxial cable) and the balanced line, to thereby make it possible to miniaturize the signal transmission circuit.
According to an aspect of this invention, a signal transmission circuit is provided for transmitting a signal received in an antenna unit to a receiver body through a glass plate. The antenna unit has an output terminal. The receiver body has an input terminal. The glass plate has first and second surfaces which are opposite to each other. The signal transmission circuit comprises a first coaxial cable having an end connected to the output terminal of the antenna unit. A second coaxial cable has an end connected to the input terminal of the receiver body. A first electrode pair consists of a pair of electrodes adhered to the first surface of the glass plate. A second electrode pair consists of a pair of electrodes adhered to the second surface of the glass plate at a position opposed to the first electrode pair. A first balanced line has an end pair connected to the first electrode pair. A second balanced line has an end pair connected to the second electrode pair. Disposed between another end of the first coaxial cable and another end pair of the first balanced line, a first balun is provided for impedance converting from the first coaxial cable to the first balanced line. Disposed between another end pair of the second balanced line and another end of the second coaxial cable, a second balun is provided for impedance converting from the second balanced line to the second coaxial cable.
The above-mentioned signal transmission circuit may be used in a global positioning system (GPS) receiver for use in a car navigation system. In this event, the antenna unit comprises a GPS antenna mounted on an outer surface of a body of a car. The receiver body comprises a GPS receiver body installed in the car. The glass plate comprises a glass plate of the car.